Electrolytic production of elemental boron



April 29, 1958 N. P. NIES ETAL ELECTROLYTIC PRODUCTION OF ELEMENTAL BOR ON Filed June 18, 1954 E M2 3 7v .6 2? 15M f We? s4r s Ziw N INVENTORSZ ELECTROLYTIC PRODUCTION OF ELEMENTAL BGRON Nelson Perry Nies, Altadena, Edgar W. Fajans, Los Angeles, and Lester L. Thomas, Lawrence E. Hiebert, and Vincent Morgan, Boron, Califi, assignors, by mesne assignments, to United States Borax & Chemical Corporation Application June 18, 1954, Serial No. 437,664

16 Claims. (Cl. 204-60) This invention has to do with the production of elemental boron by electrolysis of a suitable melt containing boron trioxide.

The invention is concerned more particularly with an improved type of electrolytic bath that requires only simple and readily available substances for its preparation.

A further aspect of the invention concerns a novel method of maintaining the effectiveness of Such electrolytic baths over long periods of operation.

A suitable melt for the electrolytic production of elemental boron from boron trioxide must be capable of dissolving the boron oxide in adequate concentration and must provide suitable fluidity and electrical conductivity. However, it has been found that those qualifications are not necessarily sufficient for practical commercial utility of the electrolytic process, since even a melt that appears satisfactory on the basis of those criteria may prove ineffective in other respects. It may, for example, produce boron of insufficient purity, or the boron deposit may not adhere properly to the cathode.

The literature includes descriptions of tests made with a rather wide variety of bath compositions for the electrolytic production of boron. For example, an article by H. H. Kahlenberg, in the Transactions of the American Electrochemical Society," vol. 47 (1925), describes a considerable number of different salts and combinations of salts from which he attempted to prepare molten baths that would dissolve B20 and permit electrolytic production of boron. The simple and stable salts mentioned by Kahlenberg include NaCl, KCl, CaCl KF, K2CO and K 50 Although small amounts of boron were obtained in a few instances, serious practical difliculties, such as poor miscibility, low conductivity, corrosive effect on electrodes or containers, or contamination of the product prevented development of any practicable process.

We have now discovered that an efiective molten bath for the electrolysis of boron trioxide can be prepared from simple salts of the type tested unsuccessfully by Kahlenberg. More particularly, we have found that potassium chloride in molten condition can be made to dissolve boron trioxide in substantially any desired proportions rates Patent '0 2,832,730 Patented Apr. 29, 1958 Although the detailed mechanism of that progressive change is not fully understood, we have found that such change can be effectively corrected, and the bath caused to operate satisfactorily over long periods of time, by treating the molten bath with acid.

For control of that acid treatment it is convenient to utilize the pH of an aqueous solution of some standard concentration, formed by solution in water of a solidified sample of the bath. For definiteness in the present description and claims, we have adopted for that purpose a solution containing 0.33 gm. of solidified melt per 100 gm. of water, and the pH value obtained fromsuch a solution is sometimes referred to as the pH of the melt. In accordance with the present aspect of the invention, acid is supplied recurrently to electrolytic baths of the type described, comprising boron trioxide, potassium fluoride and preferably, but not necessarily, a major but limited proportion of potassium chloride. The acid is added in snfiicient quantity to prevent the appearance of any appreciable alkalinity. We prefer, by such acid treatment, to maintain the pH of the bath (in the sense just defined) between about 4.5 and about 7.5.

by the addition of a suitable predetermined proportion rangeof proportions of potassium fluoride to potassium chloride, electrolytic baths of the type described typically.

change progressively with the use in a way that tends, if not corrected, to yield a less pure product. That change has been found to be accompanied by increasing alkalinity of the bath. It is believed that such alkalinity may perhaps result from hydrolysis of potassium fluoride, for example by water that may be absorbed from the atmosphere, and volatilization of the resulting hydrofluoric acid.

. For that purpose we may add to the bath any inorganic acid that does not decompose and lose its acidic nature at the temperature of the bath. A particularly effective and convenient mannerof carrying out such acid treatment is to bring into contact with the molten bath an acid that is in gaseous form at the temperature of the bath. In accordance with that aspect of the invention the electrolytic melt is contacted with an acid, as by bubbling the acid in gaseous form through the melt, to cause absorption by the melt of suflicient acid to maintain the pH of the melt between about 4.5 and about 7.5. The

acid gas may be bubbled into the hot melt through a graphite tube, the position of the tube and the rated flow of the gas being preferably such that the bubbles is already present in the bath. The acid treatment then adds no foreign molecular species to the bath composition. Whereas hydrochloric and hydrofluoric acid are preferably added directly to the molten bath in gaseous form in the manner described, hydrofluoric acid may be introduced into the bath, for example, by adding the acid salt KHF in solid form, that salt containing the acid HF, which is released at the temperature of the bath The described treatment of the bath with acid may be-carried out intermittently or continuously during electrolysis. Intermittent addition of acid has been found sutficient for maintaining effectively uniform action .of the bath. It is convenient, for example, to bring an acid gas into contact with the molten bath while the electric current is interrupted, as to replace the cathode in order torecover the deposited boron. Such addition of acid from time to time in sufficient amount to prevent the peratures such as 1400 to 1700 F., form a homogeneous melt only in proportions corresponding approximately to the region to the right of the solid line CD in .the drawing. In accordance with the invention, any point in that limited region of the drawing, so long as it represents also a suitable concentration of boron trioxide, corresponds to a composition that may be used effectively in the electrolytic production of boron.

We have discovered that concentrations of boron trioxide less than about 5% tend to give poor adhesion between the boron deposit and the cathode, while concentrations of boron trioxide greater than about 40% tend to produce too viscous a bath for convenient electrolysis. Whereas effective electrolysis may be obtained throughout the region from about 5% (line GH) to about 40% (line PQ) baths containing at least about 10% B have been found to give particularly satisfactory results. (The portion of the curve CD corresponding to that effective range of concentration of boron trioxide can be represented by a simple condition, which therefore provides a convenient criterion for defining those baths that have been found to be suitable. That condition, represented in the diagram by the curve CE, is that the concentration of potassium fluoride .in the bath is equal to or greater than about five times the square root of the percentage concentration of boron trioxide.

Compositions meeting that condition include, for example, mixtures of boron trioxide and potassium fluoxide only, with no potassium chloride. We have found such compositions to be fully effective for producing boron by electrolysis, provided the development of alkalinity is prevented. However, it is more economical to include in the melt a substantial proportion of potassium chloride, which may approach the maximum proportion permitted by the condition already defined. It is ordinarily preferred that sufiicient potassium chloride be included in the melt to maintain the content of potassium fluoride less than about 50% of the entire composition, a proportion indicated in the drawing by the line LM. In practicing the invention it is preferred, primarily for the reasons that have been given, to utilize compositions corresponding to points in the shaded area of the figure, although the region to the right of that area and between the lines GH and PQ is also capable of yielding a satisfactory product.

A full understanding'of the invention and of its further objects and advantages will be had from the following description of typical detailed procedures by which it may be carried out. Whereas those procedures represent the best mode now contemplated for carrying out the invention, it will be understood that many changes may be made in the particulars of the invention as herein described without departing from its true scope. The present description is intended as illustration only, and not as a limitation upon the scope of the invention, which is defined in the appended claims. I Electrolytic production of boron in accordance with the present invention may be carried out in a graphite crucible, which acts as anode for the electrolysis. The exterior of the crucible may be surrounded by a pr'otecfive sheath of steel or heat-resistant alloy, the whole beingmaintained in any suitable manner at the required elevated temperature. For example, the crucible maybe enclosed in a gas-fired brick furnace. The cathode may be made, for example, of iron or mild steel, and may be either flat or cylindrical in form, the latter being pre ferred since it has less tendency to warp. It is preferred to provide interior passages in the neck and bodyof the cathode through which a suitable fluid may be circulated for cooling. Such cooling may utilize air or air in which an appreciable amount of water is entrained. i We have found that it is particularly desirable to pro is used to cool the exposed portion of the cathode throughout the electrolysis; and the other set of passages 14 extends also into the main body of the cathode, and is employed for cooling just before and after withdrawal of the cathode from the bath 16 for recovery of the deposited boron. Before withdrawal of the cathode from the bath, it has been found advantageous to reduce the electric current to a low value, but preferably not to Z6I'O,'f01 a period of approximately or 10 minutes, during which the body of the cathode is cooled by a rapid how of air through the described second set of passages. The cathode is thereby considerably reduced in temperature before removal from the bath, promoting adthe bath; The entire cathode and its boron deposit then become coated with a layer of solidified melt. That layer appears to further promote adherence of the boron deposit and to provide further protection of the boron against oxidation when the partially cooled cathode is then removed from the bath. Continuation of current flow at a low rate during that cooling step helps to prevent any tendency for the deposit to become loosened from the cathode.

The portion of the cathode above the surface of the bath may be protected from oxidation by coating with solidified eletcrolyte. Such a protective coating may be produced conveniently by dipping the cool upper part of the cathode briefly into the molten bath before it is secured in operating position. The resulting coating may then be maintained throughout electrolysis by circulation of air through the described first set of passages in the upper part of the cathode. Current is preferably turned on'only after the main body of the cathode has reached substantially the operating temperature of the bath.

That operating temperature must be higher than the temperature required to produce a liquid melt. It has been found that contamination of the product with potassium is reduced if the bath temperature is higher than the temperature at which metallic potassium boils. However, the temperature should be low enough to prevent vaporization of the regular constituents of the bath. In accordance with the invention, the bath temperature is preferably maintained between about 1425 F. and

vide two sets 12 and 14 of such passages, with separate a position was then electrolyzedfor a total of 5% hours in four runs from which the boron deposit was again separately recovered. As in the other examples to be described, the resulting deposit was in each instance extracted with boiling water and with concentrated hydrochloric acid, and was analyzed chemically for percentage content of boron. The average purity of the 232 g. of boron recovered from those seven runs, made without any acidification, was 86.9%. The pH of a 0.33% solution of the solidified electrolyte at the end of each run.

was'found to increase from 7.3 to 7.6. After the described electroly sis the resulting bath was acidified by bubbling HCl gas slowly into the liquid melt for about 3 hours, reducing the pH of the described standard solution from to Electrolysisfor 1.5 hours their yielded a total of 37.7 g. of boron of average purity 93.3%. The pH at the end of that run had risen to 4.9.

Equally satisfactory results were later obtained with an electrolytic bath of composition intermediate between the two compositions just described. A bath comprising substantially 55% KCl, 30% KP and 15% B was acidified in the manner already described to produce a pH of 4.8. Electrolysis of the resulting bath during one 2 hour run yielded a total of 57.4 g. boron of average purity 94.8%, the fraction coarser than 100 mesh consisting of 29.6 g. of boron 97.3% pure.

Example 2.-An electrolytic bath was prepared by melting together in a graphite crucible substantially 40% KCl, 40% KF and 20% B 0 The bath was electrolyzed for 21.5 hours with interruptions approximately every 2 hours for recovery of deposited boron and for occasional addition of makeup. The temperature was varied between a minimum of 1495 F. and a maximum of 1595 F. without significant efiect on the operation. Current density was approximately 10 to 12 amperes per square inch, with one 4 hour run at 20 amperes per square inch. The melt was acidified initially and between runs by bubbling HCl gas into the melt for an average time of approximately 20 minutes. The pH of the melt was 5.5 after the first acidification. Although tending to increase a few tenths of a unit during each 2 hour run, the pH was gradually reduced by the intermittent acid treatment to a level of approximately 5.0. A total of 573.1 gm. boron was obtained, of which more than half exceeded 94% purity and 15% exceeded 96% purity.

Example 3.-An electrolytic melt was prepared by melting together substantially 84% KP and 14% B 0 Electrolysis during two 3 hour runs produced a well formed deposit of boron, but the purity of the product was found to be only about 85%. The pH of the bath was initially 6.85 and rose to 7.3 at the end of the second run. The melt was then treated with acid by slowly bubbling hydrofluoric acid in gaseous form into the liquid bath for 10 minutes before each of three additional runs. Although that was sufficient to lower the pH only a little, the average purity of the product increased to about 87%. Two further runs were made, preceded by similar exposure of the melt to gaseous hydrofluoric acid for 30 minutes and for 130 minutes, respectively, reducing the pH to 6.3 and then to 5.6. The average purity of the boron recovered in those two runs was found on analysis to be 89.0% and 90.7%, respectively. Whereas that purity is less than is obtainable with baths including a major (but limited) proportion of KCl, it is satisfactory for many purposes.

Example 4.-A melt consisting substantially of 44% KCl, 41% KF and 15% B 0 was electrolyzed for a total of approximately 30 hours, divided into sixteen runs for which the deposited boron was separately recovered and analyzed. The melt was treated intermittently with gaseous hydrochloric acid for irregular and relatively brief periods that averaged about 9 minutes per run, causing the pH to vary between 6.0 and 7.4. Throughout that range, boron of satisfactory purity was obtained, the average purity values for the individual runs all lying between 89.8% and 94.7%. The fraction having a grain size coarser than 100 mesh, comprising 45% of the total yield, had an average purity of 93.7% for all runs. The slight alkalinity represented by a pH value of 7.4 evidently does not prevent production of a highly satisfactory product. The relatively high purity during this run may be associated with the fact that the melt had been treated with hydrochloric acid gas prior to the electrolysis, even though that treatment did not completely eliminate the alkalinity.

Example .-A melt consisting substantially of 49.5% KCl, 27% KF and 23.5% B 0 and having a pH value of about 4.9 after acid treatment with gaseous hydrochloric acid in the manner already described, produced during 1.5 hours of electrolysis a total of 28.9 g. boron 6 having an average purity of 96.0%, the fraction coarser than mesh consisting of 14.0 g. of boron of 97.5% purity.

All of the preceding examples are given for purposes of illustration, and their particulars are not intended to imply any limitation upon the scope of theinvention, which is defined by the appended claims.

We claim:

1. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, the percentage content of potassium fluoride in the bath being at least about five times the square root of the percentage content of boron trioxide.

2. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, the percentage content of potassium fluoride in the bath being less than about fifty and being at least about five times the square root of the percentage content of boron trioxide.

3. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, the percentage content of boron trioxide being between about five and about forty, and the percentage content of potassium fluoride in the bath being less than about fifty and being at least about five times the square root of the percentage content of boron trioxide.

4. The process of producing elemental boron, which comprises electrolyzing a molten bath of boron trioxide, potassium chloride and potassium fluoride at a temperature between about 1425 and about 1700" F. todeposit boron on the cathode, the ratio of the potassium fluoride to the potassium chloride in the bath being suflicient to dissolve all of the boron trioxide at the said temperature.

5. The process of producing elemental boron, which comprises electrolyzing a molten bath consisting'essentially of boron trioxide, potassium chloride and potassium fluoride at a temperature between about l425 and about 1700 F. to deposit boron on the cathode, the percentage content of boron trioxide in the bath being between about ten and about forty, and the percentage content of potassium fluoride in the bath being sufficient to dissolve all of the boron trioxide at the said temperature.

6. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, and providing in the bath an inorganic acid that retains its acidic nature at the temperature of the molten bath to prevent the bath from becoming appreciably alkaline.

7. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, and providing in the bath while in molten condition a hydrogen halide to prevent the bath from becoming appreciably alkaline.

8. The process of producing elemental boron, which comprises producing a molten bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, electrolyzing the molten bath to deposit boron on the cathode, and contacting the bath While in molten condition with an acid gas selected from the group consisting of hydrochloric acid and hydrofluoric acid to prevent the bath from becoming appreciably alkaline.

9. The process of producing elemental boron, which comprises producing a molten bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, electrolyzing the molten bath for one period of electrolysis to deposit boron on the cathode, removing 7 the cathode from the bath to recover the deposited boron, introducing into the molten bath in absence of the cathode a stream of gaseous acid to neutralize alkalinity result ng from the electrolysis, and then electrolyzing the resulting bath for another period of electrolysis.

10. The processof producing elemental boron, which comprises producing a molten bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, electrolyzing the molten bath to deposit boron on the cathode, interrupting the electrolysis, introducing into the molten bath while the electrolysis is so interrupted a stream of gaseous acid to neutralize alkalinity resulting from the electrolysis, and then resuming the electrolysis.

11. The process of producing elemental boron, which comprises electrolyzing a molten bath comprising boron trioxide and at least a major proportion of potassium fluoride to deposit elemental boron on the cathode, then internally cooling the body of the cathode to a temperature below the melting point of the bath to form a coating of solidified electrolyte over the deposited boron, and removing the cathode from the bath while the deposited boron is protected by the said coating of solidified electrolyte.

12. The process of producing elemental boron, which comprises electrolyzing a molten bath comprising boron trioxide and at least a major proportion of potassium fluoride to deposit elemental boron on the cathode at a normal electrolyzing current, then reducing the current to a relatively low value greater than zero and internally cooling the body of the cathode to a temperature below the melting point of the bath while maintaining flow of the reduced current to form a coating of solidified electrolyte over the deposited boron, and removing the oathode from the bath While the deposited boron is protected by the said coating of solidified electrolyte.

13. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, and progressively neutralizing alkalinity developed in the molten bath by virtue of the electrolysis.

14. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, saidbath consisting essentially of boron trioxide, potassium chloride and potassium fluoride, the percentage content of boron trioxide being between about five and about forty, and the percentage content of potassium fluoride in the bath being less than about fifty and being at least about five times the square root of the percentage content of boron trioxide, and progressively neutralizing alkalinity developed in the molten bath by virtue of the electrolysis.

15. The process of producing elemental boron, which comprises electrolyzing a molten bath to deposit boron on the cathode, said bath consisting essentially of boron 'trioxide, potassium chloride and potassium fluoride, the percentage content of boron trioxide being between about five and about forty, and the percentage content of potassium fluoride in the bath being less than about fifty and being at least about five times the square root of the percentage content of boron trioxide, and providing in the bath while in molten condition an acid selected from the group consisting of hydrochloric acid and hydrofluoric acid to prevent the bath from becoming appreciably alkaline.

16. In the art of producing an oxidizable substance by electrolyzing a molten bath to deposit said substance upon an electrode that is immersed in said bath; the improvement which comprises completing the electrolysis, then internally cooling the body of the electrode to a temperature below the melting point of the bath to form a coating of solidified electrolyte over the oxidizable substance deposited on the electrode, and removing the electrode from the bath while the deposited oxidizable substance is protected by the coating of solidified electrolyte from oxidation by the atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 2,572,249 Cooper Oct. 23, 1951 

1. THE PROCESS OF PRODUCING ELEMENTAL BORON, WHICH COMPRISES ELECTROLYZING A MOLTEN BATH TO DEPOSIT BORON ON THE CATHODE, SAID BOTH CONSISTING ESSENTIALLY OF BORON TRIOXIDE POTASSIUM CHLORIDE AND POTASSIUM FLUORIDE, THE PERCENTAGE CONTANT OF POTASSIUM FLUORIDE IN THE BATH BEING AT LEAST ABOUT FIVE TIMES THE SQUARE ROOT OF THE PERCENTAGE CONTENT OF BORON TRIOXIDE. 